A clamp-on ultrasonic flowmeter is a type that measures, from outside of a tubular body, such as a water pipe, the flow velocity and the flow rate of fluid flowing inside the tubular body with an ultrasonic transducer (module for transmitting/receiving ultrasonic pulses of arbitrary frequency) attached to part of the outer surface of the tubular body. The clamp-on ultrasonic flowmeter is roughly classified into a transit time type and a Doppler type.
The transit time type uses a technique in which ultrasonic waves are made to go back and forth on a path that crosses diagonally over the fluid flowing inside a tubular body. The flow rate of the fluid is measured from the difference of time taken for the ultrasonic waves to transit the outward path and the return path, respectively.
On the other hand, the Doppler type use a technique in which suspended particles, air bubbles, or other flaws included in the fluid are presumed to move at the same speed as the fluid. The flow rate of the fluid is measured by measuring the speed of the suspended particles, air bubbles, or other flaws. Because through the transmission of ultrasonic waves into the fluid, the frequency of the ultrasonic waves reflected from the suspended particles or other flaws is changed by the Doppler effect, the flow velocity of the fluid can be calculated based on the resulting frequency drift, and the flow rate of the fluid can be calculated by subjecting the flow velocity distribution to integration computation.
Such a conventional technology for the Doppler ultrasonic flowmeter is disclosed in JP-A-2000-97742, for example. Such a Doppler ultrasonic flowmeter allows a fairly accurate flow rate measurement without touching the fluid in the unsteady state. In that reference, ultrasonic pulses (a group thereof) are transmitted toward measurement fluid subject at required intervals, and ultrasonic echoes as a result of reflection on a reflector located on the measurement line are received. Based thereon, a Doppler shift (frequency shift; change of frequency) is calculated so that a flow velocity distribution is derived for the subject fluid. Based on the resulting flow velocity distribution, the flow rate is derived by integration computation.
Referring to FIG. 6, the flow velocity distribution and flow rate computation are made as follows. A group of reflection echoes indicated by (1) are reflection echoes with respect to a specific transmission pulse, and a group of reflection echoes indicated by (2) are reflection echoes with respect to another pulse that is transmitted successively to the transmission pulse. In FIG. 6, Δt denotes a repetition period (pulse repetition period T) of the transmission pulse. The reflection echoes partially show a large amplitude at parts of A and B. The part of A indicates the reflection echoes resulting from the reflection on an entrance wall of the tube, and the part of B indicates reflection echoes resulting from the reflection on the opposite wall of the tube. The part between A and B indicates the part along the measurement line (travel path of ultrasonic waves) inside of the tube. By measuring the amount of Doppler shift of the reflection echoes at the respective positions between these parts A and B, the flow velocity of the fluid can be measured at the positions on the measurement line corresponding to the positions. In this manner, calculating the flow velocity at the respective positions can successfully lead to the flow velocity distribution as shown in the drawing, for example.
Note here that the flow velocity distribution can be derived by repeatedly executing, tens and hundreds of times, the process of calculating the flow velocity based on the received reflection echoes. Note also here that the measurement line has an angle of θf with respect to the normal to the tube pipe axis. In fact, the positions on the measurement line are converted into the positions on the cross section of the tube using the angle of θf.
The flow velocity distribution derived as such is then subjected to an integration process so that the flow rate can be calculated. At, that time, the integration process is executed not using the flow velocity distribution in its entirety, but using the flow velocity distribution of the integration range as shown in the drawing. For example, the integration range is a range from the center of the tube (tube axis) to the opposite wall.
Moreover, the specific position on the above-described measurement line (on the cross section of the pipe) is referred to as a channel. In other words, any arbitrary area of the measurement line is divided into an arbitrary number of sections, and each section is referred to as a channel. For example, when the division number is 50, there are 50 channels (which division number relates to a spatial resolution). The above-described flow velocity is thus derived on the basis of the channel, and the points of the flow velocity distribution shown in FIG. 6 respectively represent the channel position and the flow velocity thereof.
Here, the sound velocity or the distance for the ultrasonic waves to travel in the tube wall or in the fluid is known in advance. Thus, based on the known factors, in the reflection echo waveform shown in FIG. 6, calculation can be made in advance for the correspondence between the data timing and the channel position. Specifically, the time taken for transmission and reception can be calculated in advance for each of the channel positions on the cross section of the tube, and the thus derived correspondence between the channel position and the time can be stored.
As described above, the flow velocity distribution is used as a basis to calculate the flow rate through an integration process. Therefore, the integration range has a large influence over the measurement precision for the flow rate. The quantization error, however, generally occurs to the spatial resolution, which error results in an integration error, which in turn results in a measurement error.
Accordingly, there remains a need for a Doppler ultrasonic flowmeter that can more accurately measure the flow rate by lowering the measurement error. The present invention addresses this need.